TTL Logic Family

TTL Logic Family
1
Introduction
• Uses bipolar technology including
NPN transistors, diodes and
resistors.
• The NAND gate is the basic building
block
• Contains different subfamilies with
varying characteristics such as
speed and power consumption
2
Standard TTL NAND Gate
A
B
Q1
Q2
Q3
Q4
Y
L
L
sat
off
off
Off
H
L
H
sat
off
off
Off
H
H
L
sat
off
off
Off
H
H
H
iam sat
sat
On
L
3
NAND Output Current
Sinking & Sourcing
4
Sinking
Sourcing
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association with the lineage.
5
Totem pole in TTL ?
• What is totem pole?
– addition of an active pull up circuit in the
output of a gate is called totem pole. Using
Q3 and Q4 to achieve this purpose
• Why totem pole?
– To increase the switching speed of the
gate which is limited due to the parasitic
capacitance at the output.
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7
NOT Gate 7404
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NOR Gate 7402
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AND Gate 7408
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OR Gate 7432
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XOR Gate
12
Open Collector Output
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Tristate Gate
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Low Power TTL (74L)
Same as standard TTL
Except that the value
of the resistance is
increased to lower
power dissipation.
However, this is at the
expense of reduced
speed.
15
High Speed High Power
TTL (74H)
Reduced Resistors
increase power
dissipation.
Removed output
diode and added a
Darlington pair
transistor to
reduce output
resistance
16
Shottky Diode
17
Standard TTL Transistor
Switching Problem
• Trs are driven into deep saturation to fully
conduct, or cutoff to switch off.
• The result of deep saturation is that the two
junctions are now forward biased.
• The forward biasing of the BC junction
forces a large number of minority carriers
to the collector region.
• When the Tr switches off, these minority
carriers needs to be removed. This takes a
finite amount of time called the storage time
(major component of the propagation delay)
and thus increases the switch off time. 18
Solution 1
• Prevent the Tr from going deep in
saturation. This accomplished by
preventing the BC junction from
becoming forward biased.
• The Shottky diode is used to do the
above by placing it across the BC
junction. Because of its lower barrier
potential, it will conduct current from the
base directly to the collector before the
BC is forward biased. Thus less carriers
are stored in the collector area and the
switching becomes much faster.
19
SOLUTION 2
Normal Transistor
in saturation
Transistor with
Shottky diode in saturation
20
Schottky TTL (74S)
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Low Power Shottky 74LS
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Advanced Low Power Shottky 74ALS
• All saturating transistors are clamped by
using Schottky diodes.
• Inputs and outputs are clamped by
Schottky diodes
• New manufacturing and materials
reduce stray capacitance to increase
speed.
• active turn-off of the LOW-level output
transistor, producing a better HIGH-level
output voltage and thus a higher HIGH24
level noise immunity.
74ALS00
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